Antibody producing cells are the terminally differentiated cells of the B lymphocyte lineage. Throughout the differentiation process, B lineage cells display cell surface molecules which, in addition to providing specific functions, serve as landmarks for the developmental stage of a B cell or can divide B cells into functional subsets. One cell surface marker which divides mature B cells into subsets is the Lyb5 antigen. The subset of B cells expressing Lyb5 matures later than Lyb5 cells and responds to thymus independent type 2 antigens which are largely bacterial polysaccharides. The antiserum which originally defined the Lyb5 antigen was a polyclonal antiserum which, unfortunately no longer exists. Because of this lab's interest in the immune response to bacterial polysaccharides, we are particularly interested in studying the development of the Lyb5 B cell subset. To do this, we attempted to recreate the anti-Lyb5 antiserum by generating hybridomas made with spleen cells from C57BL/6 mice which had been hyperimmunized with DBA/2 spleen cells. (C57BL/6 mice have a different Lyb5 allele than DBA/2 mice.) This follows the original protocol used to generate the anti-Lyb5 specificity. Once hybridomas were established, they were screened for binding to splenic B cells from a variety of mouse strains by flow cytometry. While the anti-Lyb5 specificity has not yet been duplicated, we have identified 5 anti-B cell monoclonal antibodies of potential interest. These antibodies are being further characterized to rule out other known specificities and those with unique specificity will be studied in depth to determine the role that the cell surface molecule plays in B cell development and function.